Behaviour of pre-sintered Y-TZP during machining operations: Determination of recommended cutting parameters

Abstract Yttrium oxide Tetragonal Zirconia Polycrystals (Y-TZP) are oxide ceramics which play an important role in dental restoration. Machining of crowns and fixed partial dentures is preferably done using pre-sintered ceramics. The CAD/CAM chain is mainly automated, but the definition of cutting parameters is still challenging. Indeed, these parameters can influence the quality of the product and their selection should focus on the repeatability of machining operations. Y-TZP benefits from transformation toughening, which allows the reduction of crack growth. This material is often assimilated to a material with ductile behaviour, even though ceramics have brittle behaviour. In this paper, the NF E 66-520-5 standard, dedicated to the determination of the optimal cutting parameters suitable for ductile material, is applied to the pre-sintered ceramic Y-TZP. The objective of the standard is to calculate the specific cutting energy of the forces created between the chosen tool and material during machining operation. Specific cutting energy is the energy required to remove a unit volume of material, and this varies according to the set cutting parameters. The standard determines the upper and lower values of the cutting parameters, using graphs, to ensure repeatable machining. The resulting curves, when using pre-sintered Y-TZP and the standard application (specific cutting energy in relation to cutting speed and specific cutting energy in relation to chip thickness) are similar to the curves resulting from the use of ductile material. Indeed, curves decrease until a threshold is reached and after this point the specific cutting energy stabilises. Thus, the approach allowed the determination of the lower values of parameters for pre-sintered Y-TZP: the cutting speed (133 m/min), the minimum of the chip thickness (0.047 mm) and the axial cutting depth (0.7 mm) for a hemispherical tool in 3 mm diameter and two flutes. Over these thresholds, the machining operations are repeatable with the employed material because the specific cutting energy stabilises.